[racket-dev] contract-out

On Sun, Sep 25, 2011 at 10:55 AM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:
> Very nice! Is it easy to make a provide form that conditionally provides with contracts, such as Sam's provide-cond-contract in typed-scheme/utils/utils.rkt?

The trouble with this is that it's not clear how it should be
controlled.  There are a few obvious possibilities:

1. There's a global flag for all of Racket which turns contracts on
and off.  It should be pretty clear that this is a bad idea.
2. There's a form like `(contract-cond-out expr stuff ...)'  which
uses the contracts in `stuff' IFF `expr' is true.  This is easy to
implement, but pretty inconvenient to use.
3. Something like the status quo, where everyone defines their own
thing, but maybe with some abstraction.
4. A compiler flag.  We don't have anything like this, though,
currently, and it's not clear how it should work.


> -Ian
> ----- Original Message -----
> From: Matthew Flatt <mflatt at cs.utah.edu>
> To: dev at racket-lang.org
> Sent: Sat, 24 Sep 2011 09:41:17 -0400 (EDT)
> Subject: [racket-dev] contract-out
>
> The `racket/contract' and `racket' modules now provide `contract-out',
> which is a `provide' form for contracts. Use
>
>    (provide (contract-out ....))
>
> instead of
>
>    (provide/contract ....)
>
>
> There's one difference between `(provide (contract-out ....))' and
> `(provide/contract ....)': contract expressions in contract-out' are
> implicitly moved to the end of the module, which means that they can
> refer to variables that are defined later. For example, the following
> program works as you'd expect:
>
>  #lang racket
>
>  (provide (contract-out [mask-of (turtle? . -> . color?)])
>          turtle?
>          color?)
>
>  (define (turtle? v)
>   (memq v '(Leonardo Michelangelo Donatello Raphael)))
>  (define (color? v)
>   (memq v '(blue orange purple red)))
>
>  (define (mask-of t)
>   (match t
>     ['Leonardo 'blue]
>     ['Michelangelo 'orange]
>     ['Donatello 'purple]
>     ['Raphael 'red]))
>
>
> The `contract-out' form is implemented as a provide pre-transformer,
> which is a new kind of `provide' form that is now supported by
> `racket/provide-transform'.
>
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-- 
sam th
samth at ccs.neu.edu